Since the discovery of high critical temperature (T.sub.c) superconductors in the mid-1980s, efforts have been made to identify new high T.sub.c superconducting materials and new methods of making such materials. The term high T.sub.c superconductors, as used in this application, refers to superconductors with T.sub.c s greater than 77K, the boiling point of nitrogen. Among the known high T.sub.c superconductors are the so-called 1-2-3 materials, including YBa.sub.2 Cu.sub.3 O.sub.7-.delta. and similar materials. Other known high T.sub.c superconductors include a Bi-Ca-Sr-Cu-O family of materials.
High T.sub.c superconductors are commonly made by a solid state reaction. Conventional reactants include oxides of the superconductors' constituent elements and other compounds, such as nitrates, citrates, and carbonates, that decompose into oxides when heated. The reactants are mixed in stoichiometric quantities to form a reactant mixture, pressed to consolidate the mixture, and heated to convert the mixture into a superconductor. Depending on the composition of the reactant mixture, a multiphase, that is, a multicomponent, product rather than a single phase product may be formed. For example, a reactant mixture of 1 mole Bi.sub.2 O.sub.3, 1 mole CaCO.sub.3, 2 moles SrCO.sub.3, and 2 moles CuO has been found to form a multiphase product rather than a single phase Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8 product as expected from the stoichiometry. The presence of more than one phase in the product may have a detrimental effect on the product's properties.
One particularly important property that may be affected by the presence of more than one phase in the superconducting product is the critical current density (J.sub.c), a measure of a material's ability to carry an electrical current. A higher critical current density means that a material is able to carry a higher current than a material with a lower critical current density. Superconductors having a high critical current density are desirable for use in many of the devices that can benefit from the use of superconductors, such as motors, generators, and magnets. Because of the presence of multiple phases and other factors, such as crystallite orientation, however, many high T.sub.c superconductors have low critical current densities. As a result, the benefits to be derived from using high T.sub.c superconductors in many practical applications may be difficult to achieve.
Accordingly, what is needed in the industry are methods for making high T.sub.c superconductors having high critical current densities.